1. Field of the Invention
The present invention relates generally to stents for implanting into a living body. In particular, the present invention relates to intraluminal stents especially suited for implanting in a variety of lumens having variable characteristics, such as variable curvature, side branching, variable diameter, variable wall compliance or xe2x80x9cend effectsxe2x80x9d of either the lumen, as found, e.g., in ostia, or the stent as the parameters may change at its ends.
2. Description of the Prior Art
It is well known to use a stent to expand and impart support to different bodily conduits, such as blood vessels, by expanding a tube-like structure inside the vessel requiring support against collapse or closure. U.S. Pat. No. 5,449,373 shows a stent preferably used for vascular implantation as part of a balloon angioplasty procedure. The stent of U.S. Pat. No. 5,449,373 may be delivered through, or implanted in, a curved vessel. One shortcoming of conventional stents is that they may have deficiencies due to xe2x80x9cend effectsxe2x80x9d where the ends of the stent tend to xe2x80x9cflare outxe2x80x9d during insertion or after expansion or have a decreased radial force at the end. Still another shortcoming of conventional stents is they do not have different characteristics, (e.g., flexibility and rigidity), to accommodate any changing characteristics of the section of the lumen requiring different stent characteristics.
The present invention provides for various embodiments of an intraluminal stent which includes varied or different mechanical properties along the axial length of the stent in order to improve stent end effects, or to accommodate variable vessel features. As a result, the various embodiments of the present invention allow for variable properties such as flexibility or radial support between axial regions of the stent. These varied properties can be accomplished in a number of different ways, including decreasing or increasing the thickness or width of elements of one or more of the sections relative to other sections and/or increasing or decreasing the axial length of one or more of the sections and/or changing the cell shape and size and/or changing material properties (e.g., strength, elasticity, etc.) of the material in one section relative to other sections.
The various embodiments of the stents of the present invention may be adapted to provide more flexibility at the ends to allow the stent to accommodate the curvature of a vessel in which the stent is implanted. The degree of flexibility and the distance from the end of the stent to which the extra flexibility is imparted may be varied as specific applications dictate. This flexibility at the ends reduces the chance of a potential trauma point being created in the vessel by the stent tip pressing on the wall outside of the curve if the stent is not flexible enough along its longitudinal axis. In one embodiment of the present invention, flexibility of the stent ends is increased by reducing the gauge of the material used in a section or sections at the stent ends. In another embodiment the flexibility of the stent ends is increased by changing the dimensions of a section or sections at the stent ends. In yet another embodiment of the invention, the flexibility of the stent ends is increased by changing both the dimensions and the gauge of the material used in a section or sections at the stent ends.
The various embodiments of the stents of the present invention may also be adapted to insure increased radial strength at the ends. Radial strength is the resistance of a section of the stent, in an expanded state, to radial contraction. Increasing the radial strength of a stent at the ends is particularly advantageous for stents supporting ostia. Because lesions at an ostium tend to be more calcified or hardened, and therefore require more support, the section of the stent supporting the ostium must be relatively strong. It is also the case that a stent with uniform characteristics has a decreased radial force at the end due to the xe2x80x9cend effectxe2x80x9d whereby the last row has no support on one side. In one embodiment of the present invention, the strength of the stent at the end supporting, e.g., the ostium, is increased by reducing the length of some sections at the stent end.
The various embodiments of the stent of the present invention also reduce the chance of xe2x80x9cflarexe2x80x9d at the end of the stent while the stent is being fed into a vessel. During insertion of the catheter delivery system into a curved vessel, the delivery system, including the stent crimped on it, bend along the curvature of the vessel. This bending of the stent can cause a xe2x80x9cflaring outxe2x80x9d of the leading edge of the stent. This flaring could cause the stent to catch on the surface of the vessel which could result in trauma to the vessel, could inhibit further insertion and proper positioning in the target area, and could cause plaque to break off, which could embolize and clog the vessel. In one embodiment of the present invention, flare is minimized by making the section at the stent end stronger by reducing its length, and by making sections adjacent to the stent end more flexible by reducing their widths, thus, decreasing the bending strength of those sections. Bending strength is the resistance of a section of the stent to axial bending. As a result, the end of the stent remains tightly crimped on the balloon, and the bending moment is taken up by the deformation of the more flexible sections. Upon expansion, the reduced bending strength allows the end of the stent to curve and fit better the curvature of the vessel, thereby, reducing the pressure of the tip of the stent on the internal wall of the vessel being treated.
It is an object of this invention to provide a stent which does not have sharp points or protrusions at its end concentrating pressure on the vessel""s wall upon expansion of the stent in a curved portion of a vessel.
It is another object of this invention to provide a stent having a radial force at its distal end that is greater than the radial force in the portion of the stent proximal to the distal end.
It is yet another object of this invention to provide an expandable stent, comprising: a plurality of interconnected flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, wherein the cells disposed in the distal row of the stent are adapted to exert greater radial force and are further adapted to be more flexible than the cells disposed in the rows disposed between the distal row and the proximal end of the stent.
It is still another object of this invention to provide an expandable stent, comprising: a plurality of interconnected flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of said stent and a proximal row disposed at the proximal end of the stent, wherein the cells in the distal row of the stent and the cells disposed in the proximal row of the stent are adapted to exert greater radial force and are further adapted to be more flexible than the cells disposed in the rows disposed between the distal row and the proximal row.
It is another object of this invention to provide an expandable stent, comprising: a) a plurality of interconnected flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, each of the flexible cells comprising a first member, a second member, a third member, and a fourth member; b) a first C-shaped loop disposed between the first member and the third member; c) a second C-shaped loop disposed between the second member and the fourth member; d) a first flexible connector disposed between the first member and the second member; and e) a second flexible connector disposed between the third member and the fourth member, wherein the cells of the distal row are provided with first and third members that are shorter than the second and fourth members in the distal row, and wherein the distal row is provided with first and second flexible connectors that are more flexible than the flexible connectors in the cells in the other rows of the stent.
It is yet another object of this invention to provide an expandable stent, comprising: a) a plurality of interconnected flexible cells defining a longitudinal stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, each of the flexible cells comprising a first member, a second member, a third member, and a fourth member; b) a first C-shaped loop disposed between the first member and the third member; c) a second C-shaped loop disposed between the second member and the fourth member; d) a first flexible connector disposed between the first member and the second member; and e) a second flexible connector disposed between the third member and the fourth member, wherein the cells of the distal row are provided with first and third members that are shorter than the second and fourth members in the distal row, and wherein the distal row, and the row proximal to the distal row, are provided with first and second flexible connectors that are more flexible than the flexible connectors in the other rows of the stent.
It is a further aspect of this invention to provide an expandable stent comprising: a) a plurality of flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of flexible rows along the longitudinal axis with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, each of the flexible cells comprising a first member, a second member, a third member, and a fourth member; b) a first C-shaped loop disposed between the first member and the third member; c) a second C-shaped loop disposed between the second member and the fourth member; d) a first flexible connector disposed between the first member and the second member; and e) a second flexible connector disposed between the third member and the fourth member, wherein the cells of the distal row are provided with first and third members that are shorter than the second and fourth members in the distal row, and wherein the cells of the proximal row are provided with second and fourth members that are shorter than the first and third members in the proximal row, and wherein the distal row, and the row proximal to the distal row, and the proximal row and the row distal to the proximal row are provided with first and second flexible connectors that are more flexible than the flexible connectors in the other rows of the stent.
It is yet another object of this invention to provide an expandable stent, comprising: a plurality of flexible cells defining a stent having a proximal end and a distal end, the stent provided with means for imparting a radial force at its distal end that is greater than the radial force in the portion of the stent proximal to the distal end.
It is yet a further object of this invention to provide an expandable stent, comprising: a plurality of flexible cells defining a stent having a proximal end and a distal end, the stent provided with means for imparting a radial force at its proximal and distal ends that is greater than the radial force of that portion of the stent disposed between the proximal and distal ends.
It is another object of this invention to provide an expandable stent for treating a lumen having a unique characteristic along a portion of the lumen, comprising: a plurality of interconnected flexible cells, the cells arranged in a plurality of interconnected flexible rows defining a stent having a proximal end and a distal end and a longitudinal axis, wherein at least one of the rows is adapted to accommodate the unique characteristic of that portion of the lumen in contact with the adapted row or rows.
It is yet another object of this invention to provide a single flexible stent with a unibody or one-piece construction which is capable of imparting support to a lumen or vessel along the entire length of the stent and in which portions of the stent are adapted or modified so as to have characteristics, e.g., bending strength or radial strength, that are different than the characteristics or features in the rest of the stent along it""s longitudinal axis or about its circumference. The change in stent features will either accommodate non-uniformity in the treated lumen or may create different environmental conditions in different areas in the lumen. Non-uniformity in a treated vessel can be of many different types such as an ostium, change in diameter, change in curvature, non-continuous cross-section such as triangular or square, or non-uniformity in surface nature, etc. To accommodate such non-uniformity, portions of the stent may be adapted to provide changing dimension, flexibility, rigidity, size of cells, shape of cells, and response to pressure as dictated by specific applications. Specific applications may dictate, e.g., a desired higher radial force at one end while the other portions of the stent provide a substantially continuous support to the vessel wall with the gaps in the stent sized small enough to reduce the likelihood of tissue prolapse. Other applications may dictate a desired degree of stiffness in the center to reduce the likelihood of breakage and impart the desired degree of softness at the end to allow for the best fit with the anatomy of the target area. Other applications may dictate that one or more of the rows be provided with cells that are sized larger than the cells in the remaining rows of the stent so as to provide access to a side branch in the lumen, e.g., for introducing a second stent through one of the larger sized cells so as to permit construction of a bifurcated stent within the lumen. Still another application may dictate that one or more of the rows be provided with cells which are adapted or modified so that upon expansion of the stent the portion of the stent defined by the adapted or modified row or rows has a diameter that is either larger or smaller than the remaining portions of the stent to accommodate lumens with non-uniform diameters. One or more rows of cells may also be adapted or modified so as to have varying radial force, or varying longitudinal flexibility, or to correct for a change in properties at the end of the stent.